Patent GB-1007116-A (GB)
Title: Improvements in or relating to electrical apparatus responsive to the occurrence of a stationary value in the magnitude of an applied potential
Inventor: RAMSAY ALAN JAMES
IPC: H03K5/153
Filed: 1963-05-29
Abstract: 1,007,116. Detecting Voltage maxima and minima. HONEY WELL CONTROLS Ltd. May 14, 1964 [May 29,1963], No. 21380/63. Heading G1U.[Also in Division H3] Apparatus responsive to occurrence for stationary value in the magnitude of an applied variable electrical potential comprises a capacitance in series with switch having two operating conditions in which it is open and closed, connected across a pair of input terminals receiving the variable potential; means recurrently opening and closing the switch at predetermined frequency; and means responsive to a difference between the applied potential and the capacitance potential to derive an electric signal which changes in a predetermined manner on any occasion at which the applied potential ceases to vary after having been varying in a particular sense. A pulsating signal derived e.g. from a vapour phase gas chromatography apparatus of the kind described in Specification 1,007,114 is applied to the base of an emitter follower transistor amplifier T 1  whose emitter is corrected over series capacitor 3 to the primary of transformer 4 returned to earth through the collector/ emitter circuits of a complementary pair of symmetrical transistors T2a, T2b operating as an electronic switching circuit controlled by a 50 c.p.s. square switching wave applied from source 6 in antiphase to the bases. The secondary of transformer 4 excies a 4 stage transistor squaring amplifier T 3  to T 6 ; the output being taken from the collector of T 6  and applied to the base of transistor T7 connected with transistor T8 as a phase sensitive gate circuit excited from source 6. The emitter of T8 is earthed, the collector of T8 and the emitter of T7 are interconnected and the collector of T7 is connected to point 11 which is the negative output of a half wave rectifier circuit comprising capacitor 12, resistor 13, and diode 14 excited by a 50 c.p.s. sinusoid from source 6. Point 11 is also connected to a transistorised Schmitt trigger circuit 15 whose output is differentiated at 17 and applied to output terminal 15; the interconnection of resistor 13 and diode 14 being returned to the base of transistor T8 over resistor 18. A positive supply is applied to the collector of transistor T1 and a negative supply to its emitter circuit and to the spring circuit T 3  to T 7 . In operation,transistors T2a,T2b are alternately both conductive and both nonconductive, so that on alternate half cycles of the excitation, pulses of current flow in the primary of transformer 4 of sense dependent on the sense of variation of the input signal, so that the input from T 3  to the squaring amplifier is a square wave whose phase varies 180 degrees in dependence on decrease or increase of the input signal to the circuit, and is zero when the latter is constant. The corresponding output of the squaring amplifier energizes the base of transistor T7 and the reference sinusoid at diode 14 is phased to the excitation of the switching circuit J so that negative excursions are coincident when the input signal is decreasing, and are antiphased when the signal is increasing. Transistor T7 derives operating potential from point 11 which is negative to earth and conducts during negative half cycles of its base signal. Capacitor 12 is of such value that if T 7  conducts during negative halfcycles of the sinusoid energizing diode 14, its charge decreases, so that no negative potential appears at point 71 when the input signal is decreasing while the input signal is stationary or increasing, T 7  is either nonconductive or conductive only in half cycles which are opposite to those during which capacitor 12 is charged over diode 14. Transistor 18 is energized by an unsmoothed half-wave rectified voltage to conduct only during negative halfcycles of the reference sinusoid, and T 7  can only conduct with T 8  so that the potential at 11 is held negative when the input signal is increasing or stationary but rises to earth potential when the input is decreasing. The trigger circuit 15 changes from first to second state as the potential at 11 changes from negative to earth and vice versa, and its output is differentiated at 17 to pulse at each change of state, with diode suppression of the pulses at change from the first to the second state so that pulses appear at output 16 when the input signal has been decreasing and ceases to do so; i.e. at a minimum or a decreasing point of inflexion. The circuit incorporating T 7 , T 8 , 15, 17 and 14 may be duplicated with phase reversal of the reference voltage to indicate a maximum or an increasing point of inflexion of the input signal. A modified gating circuit utilizing a symmetrical function transistor may be employed (Fig. 2, not shown). A simplified version of the circuit is described in the Provisional Specification.